The present invention concerns a method and apparatus for deriving information from physical events.
There are many events from which it is desirable to have measurements but which are not susceptible to direct measurement or in which measurement of individual events can only be carried out with very great difficulty. A particularly important class of such events is the flow of particles in a stream of gaseous or liquid fluid. Systems of such particle flow include smoke in columns of air, unwanted particles in liquid systems such as high pressure hydraulic systems, and biological particles which can indicate the presence of bacteria or disease in urine. Measurement of all of these systems provides substantial problems in that the size of particles can vary, the velocity with which the particles are travelling can vary and the number of particles in any one unit of volume can also vary. Additionally the times of arrival of individual particles into the confines of the measurement apparatus cannot be predicted exactly and the shape and physical nature of the particles can vary. All these factors perturb the final measurement.
Nevertheless the detection and measurement of particles in flowing systems is frequently of great importance. One of the examples already given relates to the measurement of bacteria in urine. Accurate measurement of the size of such bacteria particles can give very good indications as to the presence or not of certain diseases. In high pressure hydraulic fluids involving filtering the breakdown of filters can cause catastrophic results and the measurement of particles in the hydraulic flow can provide an early indication as to the efficiency of the filter system.
As a result of these demands for measurement systems a number of particular sizing techniques have been developed. Some of these are based on Doppler methods and require the interferometric combination of crossed laser beams to create a structured pattern. This requires coherent laser light sources and precision lasers, or more recently the use of defraction gratings. The extent of the structured light field necessarily occupies a large part of the inspection volume and consequently requires quality optical components. An example of such a technique is disclosed in U.S. Pat. No. 4,854,705.
An example of a heuristic approach in which a more direct attempt is made to measure the individual sizes and velocities of particles in a flowing stream is described in International Patent Application No. WO93/16368. In this specification a flow of particles is passed through a cell and a structured monochromatic light field is projected into the cell. The particles pass transversely and successively through the spaced variations of the light field, the spacings of which are set in accordance with the expected range of particle size. Variations in light intensity caused by the passage of the particles relative to the light field are detected and the size of a particle can be calculated by plotting the mean peak signal of the sensor as a function of the normalised peak-to-trough variation in the output pulses generated by the passages of the particle through the light field. Such a system can be made in an extremely compact and relatively inexpensive manner but is not suitable for relatively large flow sizes where there are likely to be a substantial number of particles in the volume where the measurements are being made. Thus this system is not suited, for example, measuring the distribution of particles in the situation where it is required to provide measurements of smoke particles in a gas flow.
Thus the present invention is concerned with providing a solution to the above problems and in particular a solution to the problem of providing accurate measurements of multiple physical events which are not directly observable.
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